The present invention relates to a process and a unit for the hot- and cold-rolling of ferrous and non-ferrous metals for the manufacture of a round bar or wire rod from a bar or round wire rod having a larger diameter.
As is known, metal rolling generally uses rolling mills which comprise a plurality of stands or rolling units arranged in a line in order to progressively reduce the diameter of the bar in input.
It is furthermore known that on a rolling stand it is practically impossible to obtain a bar with a round profile directly from a round-profile bar having a larger diameter. More precisely, if one feeds a round-profile bar to a rolling stand which has rolls shaped so as to provide a round profile, the output result is not a bar with a round profile but a lobed-profile bar which is not acceptable for the market.
Due to this reason, in a rolling line with rolling units having rolls shaped so as to generate a round profile, one alternates rolling units having rolls shaped so as to generate a non-round intermediate profile which is studied so that during the subsequent passage through a rolling unit with round-profile rolls one actually obtains a round-profile bar which can be accepted by the market.
Currently, in order to obtain rolled products with a round profile, rolling mills of two types are mainly used: rolling mills with two-roll stands and rolling mills with three-roll stands.
In rolling mills with two-roll stands, stands with two rolls shaped so as to provide a round profile are generally alternated with stands with two rolls shaped so as to provide an oval profile. FIGS. 2 to 5 illustrate the sequence of the cross-sections of an initially round bar (FIG. 1) in output from the various rolling stands in rolling mills of this type.
In rolling mills with three-roll stands, stands with three rolls which are arranged so that their axes define the sides of an equilateral triangle and are shaped so as to provide a round profile are alternated with stands with three rolls which are arranged in a similar manner and have a cylindrical skirt in order to define a profile having an almost triangular cross-section. FIGS. 7 to 10 illustrate the sequence of the cross-sections of an initially round bar (FIG. 6) in output from the various rolling stands in rolling mills of this type.
In any kind of rolling mill, the rolls of the various rolling stands are actuated so that they rotate about their axes by means of one motor for each stand, or by means of a single motor which, by means of a speed reduction unit, actuates the rolls of the various stands so that the rotation rate of the rolls complies with the well-known laws of constant rates of metal flow in the various rolling stands which cooperate to deform the bar starting from its entry in the rolling stands up to its exit.
The use of these rolling stands shows problems particularly when it is necessary to pass from the production of bars of a certain diameter to bars having a different diameter.
In this case, in fact, with conventional rolling stands it is necessary to replace the sets of two or three rolls of the two final rolling units and of their guides and adapt the rotation rates of the rolls, for example by replacing some components of the reduction unit which is connected to the rolls or by means of electric adjustments. These operations entail relatively long rolling stand downtimes which make it economically unprofitable to perform small-scale production of profiles whose diameter differs from the "standard" diameters which can be obtained in output from the rolling stands usually used.